This invention relates to promoted absorbents and their preparation and more particularly to antimony pentoxide-promoted porous absorbents and their preparation and use in the removal of metal contaminates from hydrocarbon products.
The presence of even trace amounts of metals such as copper and iron in hydrocarbon products can be highly deleterious. An example is found in metal contaminated middle distillate fractions, such as fuel oils involving kerosene, jet fuel, and diesel fuel. Contaminates, such as small amounts of copper and iron, can be present in such hydrocarbon fractions from a number of sources. For example, in the purification of hydrocarbon fractions designated for use as a jet fuel or as a diesel fuel, copper and/or iron-based catalysts can be used to remove offensive thiohydrocarbons, such as mercaptans, to produce a product stream of a xe2x80x9csweetenedxe2x80x9d hydrocarbon fuel. Subsequent to the sweetening process, the product stream can be subjected to an absorption procedure to remove trace amounts of copper and iron from the hydrocarbon fuel. A suitable absorption process can involve passing the sweetened product streams through a bed of an absorbent which absorbs the contaminated copper or other metal from the stream. An aluminum silicate absorbent, such as attapulgite, bentonite, kaolinite, halloysite, or the like, may be used. In addition to an aluminum silicate clay absorbent, large pore aluminosilicate zeolites, such as zeolite Y, may also be used to absorb trace metals from the stream.
In accordance with the present invention, there is provided a process for the preparation of a promoted absorbent effective for the removal of metal contaminants from hydrocarbon products. In carrying out the invention, there is provided a porous absorbent substrate in which the predominant surface area is contained within the internal pore volume of the substrate material. The absorbent substrate is contacted with a solution of an absorption-promoting agent to provide a mixture of the absorbent substrate with the solution of promoting agent to at least partially fill the internal pore volume of the substrate with the solution of promoting agent. The mixture of the particulate substrate and the promoting agent solution is agitated for a time sufficient to effect distribution of the promoting agent within the internal pore volume of the absorbent support material. Thereafter, the support substrate is contacted with a solution of the promoting agent in an amount, when added to the solution of promoting agent previously applied, to provide an amount of the promoting agent solution which is at least equal to the pore volume of the porous absorbent substrate. This mixture is agitated for time sufficient to effect distribution of the promoting agent within the internal pore volume of the support material, and the absorbent substrate material is then dried at a temperature sufficient to dehydrate the substrate material with the promoting agent in place. The promoting agent is an oxide of a metal of Group 15 (new notation) of the Periodic Table, which metal comprises antimony or the Group 15 metals adjacent to antimony, specifically arsenic or bismuth. Preferably, the Group 15 metal is antimony.
In a preferred embodiment of the invention, there is provided a process for the preparation of an antimony pentoxide-promoted absorbent. In carrying out the invention, a particulate porous absorbent substrate is treated with an aqueous solution of antimony pentoxide, typically having an antimony pentoxide concentration of about 10 wt. % or more. The porous absorbent substrate has an average particle size within the range of 1-5 mm and an average pore volume within the range of 0.7-0.85 and preferably within the range of 0.75-0.8 cubic centimeters/gram. At least 80% of the surface area of the support is contained within the internal pore volume of the absorbent. The absorbent support is contacted with the antimony pentoxide solution, or sol, in an amount equal to or in excess of the pore volume of the porous absorbent support. The treating solution can be applied to the porous absorbent support in a single treatment, but preferably the treating solution is applied in at least two stages. Whenever multiple application stages are employed, after the initial stage, the mixture of the particulate absorbent and the antimony pentoxide solution is agitated in order to effect a distribution of the antimony concentration within the pore volume of the support. Thereafter, the absorbent material can be contacted in one or two additional stages with the antimony pentoxide solution in a total amount, including the initial step, equal to or in excess to the pore volume of the porous absorbent support material. The absorbent material is then dried at an elevated temperature sufficient to dehydrate the support.
In a further aspect to the present invention, there is provided a process for the removal of a trace metal, such as copper or iron, from a hydrocarbon product contaminated by the trace metal. In carrying out this aspect of the invention, the hydrocarbon product is contacted with an absorbent material comprising antimony pentoxide supported on an absorbent substrate. The hydrocarbon product is then withdrawn from the absorbent material to provide a purified hydrocarbon product in which at least 99.5 wt. % of the trace metal in the feedstock is removed providing a final product having a trace metal content of no more than 0.5 wt. % of the trace metal content of the feed product. In a preferred embodiment of the invention, the trace metal comprises copper such as may be present due to a sweetening process carried out over a copper catalyst. Preferably the substrate material is a fired attapulgite clay with a water content of about 5.0 wt. % or less.
The purified product preferably exhibits a copper content of about 0.03 ppm or less, as indicated by experimental work described below. The copper content of the purified product may be reduced to a value down to about 0.01 ppm, and it is preferred in carrying out the invention to provide a copper content within the range of about 0.01-0.03 ppm over a prolonged portion of use of the promoted absorbent.